Cynthia Steward walked into the store just as a customer was hitting full-throttle in an attempt to badger the owner into taking a sale item back for a full refund without a receipt. A group of customers who had been waiting to pay for their purchases had backed away from the counter, eyes averted. They looked uncomfortable and nervous.

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When the owner found and printed out a copy of the original receipt proving that the item had indeed been sold at a discounted price, the agitated customer blew up. “You’re just trying to rip me off,” she screamed as she snatched her purchase off the counter and stormed toward the door. “I can’t believe you treat customers this way. I’m never coming back!”

As the door slammed, there was a collective sigh of relief and a ripple of nervous laughter. Steward knew exactly how everyone felt. Even though none of the vitriol had been directed at her, the 29-year-old manager from Quinton, N.J., had cringed as she heard the customer berating the store owner.

Rudeness, even if it’s not aimed at us, can derail a day. It can spoil a meal and ruin a good mood. It can hamstring creativity and hamper job performance. It makes us feel uncomfortable and conflicted: We don’t want to be involved, but we feel like we should be.

As it turns out, people can be so distressed by rudeness they’ll stop patronizing a business after witnessing one worker berate another, researchers reported in a study published in the August issue of the Journal of Consumer Research.

To see how employee rudeness affects consumers, the researchers set up an experiment in which volunteers were shown one of three videos. In one, the manager of a well-known bookstore chain nastily scolds a cashier for talking too long on the phone with a friend and making customers wait. In another, the manager politely asks the cashier to hang up and take the next customer. In the third, there is no manager, just an incompetent cashier.

People who saw the rude interaction got angry at the store, says the study’s lead author Christine Porath, a professor who's now at the McDonough School of Business at Georgetown University. “They generalized about the employees who worked for the firm and the firm itself,” Porath says. “They were far less likely to continue to patronize the firm. They didn’t want to give it money.”

Rudeness offends people’s sense of justice, says Porath. And it’s very distracting to watch. As proof of that, she points to an earlier study in which she and her colleagues showed that people get so disturbed when they witness an episode of rudeness that it measurably affects their creativity and performance.

In that study, volunteers were asked to solve some anagrams and to figure out the solution to another type of problem by brainstorming. Then the volunteers were shown a video of a supervisor berating a subordinate. When the volunteers were asked to brainstorm and to solve anagrams a second time, their performance was markedly worse.

That makes a lot of sense to Dan Baugher, associate dean and director of graduate programs at Pace University’s Lubin School of Business. If the rudeness is directed towards us, it can feel like a violation, he says. If it’s directed toward someone else, we feel torn.

“Most people want to avoid conflict,” Baugher says. “But we feel anxiety if we don’t do anything.”

What we’re watching when we see rudeness is an act of aggression, says Pier Forni, a professor at Johns Hopkins University and co-founder of the Johns Hopkins Civility Project.

“Rudeness is very traumatic for those at the receiving end of it,” Forni says. “But it can also be traumatic for those who are just witnessing it.”

We’re wired up with mirror neurons, which make us feel empathy, Forni explains. When we watch someone being rude, our feelings of empathy run up against our desire to avoid conflict. Small wonder that we feel bad afterwards.

The impact of witnessed incivility may go far beyond consumers’ preferences and an individual’s anxiety, a Scottish researcher suggests.

Rhona Flin, a researcher from the University of Aberdeen, is currently looking at the fallout from rudeness in the health care setting. In a recent editorial in the British Medical Journal, Flin discussed the prevalence of rudeness in the operating room.

She pointed to a recent survey of operating room staff that found that 66 percent of workers said they had “received aggressive behavior” from nurses, while 53 percent had suffered similar treatment from surgeons. A large number of those interviewed — 63 percent — reported disagreements between surgeons and nurses in the OR.